Tested in single player in the end dimension. Happens on version 1.20.

Endermen can spawn on waterlogged top slabs.

How to recreate:

1. Go to the end and make a platform of waterlogged top slabs far from any end islands or underneath an island.
2. Endermen should start spawning immediately. You may need to head away a bit to get endermen to despawn from the main island so they can spawn on the platform.

If you leave and come back to make sure they didn't spawn while constructing the platform they will spawn again. To test if they teleported to the platform place a block (I used netherack) a few blocks from the platform and anger the enderman that spawns on it. They won't be able to teleport to the platform (picture of my test attached).

What I expected to happen:

Endermen shouldn't be able to spawn on any blocks they can't teleport to, so they should not be able to spawn on waterlogged top slabs.

I know this is a duplicated bug report (MC-205603) but when I commented on the other report nobody came to check it, and with everything that's happened on the Minecraft subreddit I don't want to participate in it.